There is a type of hearing impairment which is normal hearing in one ear, and impaired hearing in the other ear. This is called unilateral hearing loss herein.
With patients suffering from unilateral hearing loss, CROS (contra-lateral routing of signals) hearing aids are used with which a microphone picks up input sound coming from the impaired hearing side, sends it to the hearing aid worn on the normal hearing side, and the sound is reproduced on the normal hearing side. A variation on the CROS hearing aids theme are BICROS hearing aids, with which a microphone is used not only on the impaired hearing side, but also on the normal hearing side, and input sounds from the microphones at both ears are combined and outputted. The BICROS hearing aids are suitable for bilateral hearing loss (see Non Patent Citation 1, for example).
Furthermore, to give a sense of sound source direction and a sense of hearing that is close to that of a normal hearing side to the patient who has a hearing level difference between the left and right ears, there is a technique in which a tiny differential in left and right microphone inputs is decided, and an audio band pass filter is applied to signals obtained by amplifying these two input signals with a differential amplifier (see Patent Citation 1, for example).
Also, in an effort to improve hearing equilibrium between the ears in a deaf patient with a difference in hearing level between the left and right ears, there is a technique in which the hearing level difference and time difference between the left and right ears is measured from an audiogram of a deaf patient, the nonlinear amplification characteristics are varied for each frequency band with respect to the input signal on the normal hearing side, and the time delay is varied for each frequency band, to produce an output signal (see Patent Citation 2, for example).
Further, there is a technique in which signals from both ears are analyzed to estimate the sound source direction, and a sound signal processor suppresses the sound signal in a specific direction or emphasizes it, or a technique in which signals from both ears are analyzed to estimate the amount of masking, and masking is improved (see Patent Citation 3, for example).